Lead-acid recombination batteries offer a number of advantages compared to flooded cell batteries. Flooded lead-acid batteries are designed to have an excess of electrolyte that floods the cell container, completely saturating the plate group and extending into the head space above the plate group to provide a reservoir. The electrolyte reservoir is necessary because as the battery is charged, water in the electrolyte is electrolyzed into oxygen and hydrogen gases, which escape from the cell and deplete the electrolyte volume. To make up for the loss of electrolyte, water must be periodically re-introduced into the cell, or the reservoir must be made large enough to compensate for the expected loss over the life of the battery. Thus, there is a certain minimum amount of maintenance required for flooded cell batteries.
Recently, valve-regulated lead-acid (VRLA) recombinant batteries have been introduced that are suitable for, among other things, deep discharge applications. The electrolyte in VRLA batteries is absorbed in separators positioned between the positive and negative plates. VRLA batteries rely upon internal gas recombination to minimize electrolyte loss over the life of the battery, thereby eliminating the need for re-watering. Internal gas recombination is achieved by allowing oxygen generated at the positive electrode to diffuse to the negative electrode, where it recombines to form water and also suppresses the evolution of hydrogen. The diffusion of oxygen is facilitated by providing a matrix that has electrolyte-free pathways. The recombination process is further enhanced by sealing the cell with a mechanical valve to keep the oxygen from escaping so it has greater opportunity for recombination. The valve is designed to regulate the pressure of the cell at a predetermined level, hence the term, "valve-regulated".
There are two commercially available technologies for achieving the enhanced oxygen diffusion. One technology makes use of a gelled electrolyte. In gel technology, the electrolyte is immobilized by introducing a gelling agent such as fumed silica. Gas channels form in the gel matrix in the early stages of the cell's life as water is lost via electrolysis. Once the gas channels are formed, further water loss is minimized by the recombination process. Unlike the AGM matrix, the gel matrix keeps the electrolyte immobilized and there is little bulk movement.
The other technology for enhancing oxygen diffusion makes use of a fibrous mat material (FMM) separator between the electrodes. A widely used FMM for this purpose is an absorbed glass mat (AGM). The AGM is a non-woven fabric comprised of glass micro-fibers that retain the electrolyte by capillary action, but also provide gas spaces as long as the matrix is not fully saturated with electrolyte. The electrolyte is still free to move within the matrix, but is more confined than in a flooded cell. Another fibrous material gaining acceptance is a non-woven mat constructed from a polymeric component such as polypropylene or polyethylene.
The performance of VRLA batteries using FMM separators may be degraded over time for several reasons. These include short circuits between battery cell plates. The short circuits can occur along exposed plate side walls and also by dendritic short circuits between plate surfaces. This problem is exacerbated if the edges of the fibrous mat separator material are not aligned in close registration with the edges of the plate. Misalignment can occur during battery manufacture as the plate-fibrous material combination is inserted into temporary containers called "burning boxes" for casting of internal plate straps. This procedure is described in U.S. Pat. No. 4,114,260 to DiGiacomo et al., the content of which is incorporated herein by reference. This problem has been addressed in the past by providing reinforcing straps and films to hold plates and fibrous material in registration. This approach is expensive and requires additional manufacturing steps.
These problems and the advantages of the present invention will be better appreciated with an understanding of the previously used approach for installing fibrous separator material such as an AGM between plates of a VRLA battery cell. FIG. 1 is a schematic representation of a typical VRLA battery cell plate 20. In this instance, the plate has a generally rectangular shape and includes a lug 22 which is connected electrically to a battery terminal using some type of cast strap (not shown). The plate also includes vertical side walls 20a, 20b and a bottom edge 20c. The plate further includes opposing flat faces.
The AGM material has been wrapped around the plate as shown in FIG. 2. A sheet of AGM material 26 is wrapped around the bottom edge 20c of the plate 20 and extends upwardly along the opposing plate faces towards the top of the plate. This approach leaves both vertical side walls 20a, 20b exposed and more susceptible to plate-to-plate short circuits. Moreover, it will be readily appreciated that this wrapping technique leaves two exposed, loose edges of AGM material that may be difficult to keep aligned with the plate itself during assembly of the battery cell. The assembly of the battery cells for some applications is accomplished by hand. This is particularly true for cells used for motive power applications. Maintaining the proper alignment between the plates 20 and the AGM material 26 is difficult as the plate-AGM combination goes through a series of battery cell manufacturing steps. This problem has been addressed in the past by incorporating some type of sheath or restraining means surrounding the AGM material. The resulting extra cost and additional manufacturing steps are disadvantageous.
There is a need then for a method of installing fibrous mat separators in VRLA battery cells so as to minimize plate-to-plate short circuits and to ensure alignment between the separators and the plates. The present invention addresses this and other needs. Further, the present invention provides additional advantages and solutions to additional problems not necessarily stated herein. The scope of the present invention includes those advantages and solutions to these additional problems.